Environmental Chemical Behavior And Mechanism Of VOCs And PM_2.5

As important reactive compounds,VOCs contribute significantly to O₃ and secondary aerosol generation.Conducting targeted research on the secondary generation and impact mechanism of PM_2.5 and O₃,interface environmental chemistry of PM_2.5,and chemical activity and source analysis of VOCs is the foundation for achieving the goal of“promoting coordinated control of PM_2.5 and O₃”proposed in the 14th Five-Year Plan and the 2035 Vision Outline for China’s National Economic and Social Development.Hohhot,the capital city of Inner Mongolia Autonomous Region,as well as a central city in the Chinese boundary regions facing severe seasonal air pollution.It is a typical northern city with coal-fired energy as the main source,facing severe seasonal air pollution problems with PM_2.5 in winter and O₃ in summer as the primary pollutants.Accordingly,this work carried out a deep study on the chemical behavior and mechanism of atmospheric VOCs and fine particulate matter in Hohhot city in response to the significant seasonal pollution characteristics of both VOCs and PM_2.5.This study clarified the spatial characteristics of atmospheric VOCs,analyzed the species-based photochemical activity of VOCs and their sources and initially clarified the priority species and priority sources of VOCs control in the city.The patterns and trends of dominant species in haze intensification are clarified,the vital mechanisms of secondary fine particulate matter explosive growth are revealed,and the key interfacial chemical reactions of SIA on the surface of fine particulate matter under different background conditions are also derived.In addition,seasonal characteristics of fine particulate matter acidity were also investigated in this work,and the mechanisms of both meteorological elements and chemical components on fine particulate matter acidity were also elucidated.This paper is of great practical significance for clarifying the causes of air pollution and further refining the scientific control of regional air pollution in Hohhot,as well as providing important theoretical support for the continuous improvement of air quality in Hohhot and its corresponding city cluster.The main findings were as follows,1.Demonstrate the seasonal air pollution characteristics at Hohhot.PM_2.5 and O₃ are the primary pollutants limiting the air quality in winter and summer,respectively.Dust periods occurred occasionally in spring and autumn.2.Calculate the species-based chemical activity of VOCs,analyzed VOCs sources,transport pathways and their corresponding seasonal patterns.Continuous observation results of VOCs among four seasons indicated that alkanes and OVOCs shared the more than 60%of the annual average TVOCs.The seasonal distribution characteristics of TVOCs are mainly governed by OVOCs,alkanes and alkenes.The diurnal distribution characteristics of TVOCs are mainly regulated by emissions characteristics,free radical concentrations,and chemical activity of VOCs subgroups.Both OFP and L^OH results showed alkenes and OVOCs were the dominate contributors to O₃,especially propylene,ethene,isoprene and aldehyde,respectively.Source contributions of VOCs exhibited significant seasonal patterns.Combustion（45%）,industrial（15%）and background（27%）sources drove VOCs sources during heating seasons,while solvent（39%）,biogenic（20%）and industrial（17%）sources leaded the VOCs sources among non-heating seasons.Potential pollution sources were located at eastern Bayannur,northern Ordos and entire Baotou,and the impact of long-distance transportation was not negligible.3.The key chemical reactions and the stoichiometric characteristics of chemical components in the generation of fine particulate matter at different pollution stages are clarified,the decisive role of aerosol liquid water（ALW）is explained,and the mechanism of fine particulate matter generation in the haze process is deeply portrayed among stages.Secondary ion composition varied from（NH₄）₂SO₄ and NH₄NO₃ dominant to（NH₄）₂SO₄,NH₄NO₃ and NH₄Cl co-dominant.The results of the SIA generation mechanism indicated that the significant increase of both reactive gaseous pollutants and relative humidity broke the equilibrium of gaseous pollutants between gas phase and particulate matter"micro-droplets"and thus induce their heterogeneous chemical reactions,resulting in the outbreak and intensification of haze.The key chemical reaction on secondary sulfate generations were(（1）+₂(（6）+₃(（6）+₂→(（1）+₄⁺+₂^-+⁺+₃^-and chemical behavior of both NO₂（aq）and NH₃（aq）varied significantly during different pollution stages.Besides,the chemical regimes of reactive gases transferred from HNO₃-sensitive regime to HNO₃ and NH₃ co-sensitve regime.4.The seasonal characteristics of fine particulate matter acidity were investigated,mechanisms of meteorological elements and chemical components on fine particulate matter acidity were modelled to clarify the dominant factors of fine particulate matter acidity and the interaction mechanisms between different factors were also found.PM_2.5 acidity in Hohhot displayed a"double-peak"characteristic and its peak values varied significantly among seasons.The results of the sensitivity test of meteorological elements on PM_2.5 pH showed that temperature contributed the most and followed by relative humidity.Sensitivity test of chemical composition on PM_2.5 acidity showed that sulfate had the strongest impact on PM_2.5 pH,followed by TNO₃,TNH_x and TCl,respectively.Sulfate adjusted PM_2.5 pH by adjusting the generated sulfate and bisulfate to change the H⁺concentration;TNO₃,and TNH_x adjust PM_2.5 pH by adjusting the gas-particle distribution of the semi volatile components.The surge of crustal elements during dust storms had a significant effect on the elevation of PM_2.5 acidity.